Livestock confinement facilities generate large amounts of animal waste that can create serious environmental and human health concerns. For example, animal waste constituents such as organic matter, nitrogen, phosphorus, pathogens and metals can degrade water quality, air quality, and adversely impact human health. Organic matter, for example, contains a high amount of biodegradable organics and when discharged to surface waters will compete for, and deplete the limited amount of dissolved oxygen available, causing fish to die and other undesirable impacts. Similarly, nutrient loading from nitrogen and phosphorus can lead to eutrophication of surface waters.
The annual accumulation of organic waste in the world is immense. There are approximately 450,000 Animal Feeding Operations (“AFOs”) in the United States. Common types of AFOs include dairies, cattle feedlots, and poultry farms. A single dairy cow produces approximately 120 pounds of wet manure per day. The waste produced per day by one dairy cow is equal to that of 20-40 people. If properly stored and used, manure from animal feeding operations can be a valuable resource.
Anaerobic digester technology is a manure management technology capable of alleviating environmental concerns through waste stabilization, odor reduction, pathogen control and greenhouse gas entrapment and mitigation, while producing a renewable source of heat and power (US-EPA, 2005). Adoption of anaerobic digesters on US dairies is growing but still slow with numbers insufficient to meet the agreement between the US and its dairy industry to reduce climate impacts from dairies by 25% by 2020 (USDA, 2010). An important concern in the adoption of anaerobic digester technology resides in the fact that anaerobic digester units do not recover nutrients. This is important because dairy Commercial Animal Feeding Operations (CAFOs) experience nitrogen and phosphorous overloads of 36% and 55%, respectively (USDA-APHIS, 2004).
Impacts of potential farm overloads express themselves in many air and water quality threats. High concentrations of ammonia can result in odors and can also interact with other air constituents to produce particulate matter (PM25) (US-EPA, 2004), which is detrimental to human health. The U.S. agriculture industry is dependent on nitrogen-based fertilizers, which in turn is dependent on natural gas as the primary source of hydrogen to yield ammonia during the nitrogen-fixing Haber process. Clearly, technologies or mechanisms capable of collecting and concentrating existing, under-utilized forms of nitrogen, such as that present in manure waste streams, could play an important role in diminishing concerns that exist with inorganic fertilizer production.
Within water quality, leaching and excessive land applications are capable of transporting nitrogen and phosphorous compounds to the ground and surface water. Ionic ammonia and its inorganic derivatives, nitrite and nitrate, are harmful to both human and aquatic animals, with ammonia being toxic to fish, nitrite being a known carcinogen, and nitrate capable of causing blue baby syndrome and pregnant miscarriage (WS-DOH, 2005).
Phosphorous has long been implicated as a major contributor to water body eutrophication. Concerns regarding peak phosphorous levels potentially outweigh concerns associated with energy costs with nitrogen fertilizer. Numerous reports have shown that rock phosphate reserves could be exhausted during the next 50-100 years. In addition, sources will be tied to a few particular countries with quality of product diminishing, and cost of extraction increasing (Smil, 2000). With animal manures typically containing phosphorous:nitrogen ratios two to three times the normal ratio required for crop fertilization, it is easy to see why CAFOs struggle with phosphorous-loading to fields. However, concentrated sources of phosphorous and nitrogen, such as those available on CAFOs, could represent a viable source for recycled phosphorous, if economically viable technologies could be commercialized, which could potentially delay concerns regarding availability and demand.
These environmental threats can be, in part, diminished through incorporation of nutrient recovery technology capable of concentrating and exporting nutrients from the farm. Nutrient recovery also allows for wider adoption of bio-based fertilizers, replacing, at least in part, the demand for fossil-fuel based fertilizers and all of the climatic/environmental concerns associated with their production.
Several traditional wastewater technologies exist for the control and recovery of nutrients from human and industrial wastewater, however, these technologies are not cost effective or reliable when applied to manures within a farm environment. Thus, a need still exists for methods and apparatuses to recover nutrients from anaerobic digester waste material.